Sorption of Pb(II) from Aqueous Solutions by Acid-Modified Clinoptilolite-Rich Tuffs with Different Si/Al Ratios
, Atl V. Córdova Quiroz 2, María T. Olguín 3,*, América R. Vázquez-Olmos 4, Joel Vargas 5, Francisco Anguebes-Franseschi 2 and German Giácoman-Vallejos 6Round 1
Reviewer 1 Report
The manuscript presents a very systematic work of adsorption using zeolites. Authors do not provide a strong conclusion to highlight the meaningfulness of this study. Results are barely discussed and explanations provided are not sound. Several comments follow:
1) Tittle must highlight that the study is conducted with adsorption removal using zeolites: Si/Al ratio in natural zeolite clinoptilolite
2) Identify clinoptilolite as natural zeolite in the abstract.
3) L58-61: Can this differential structure be shown in a figure?
4) L61-62: Why it presents selective adsorption of metal cations?
5) Introduction presents a too general overview on zeolite materials. Authors are encouraged to define a clearer outline that guides the reader to the actual novelty of this research contribution.
6) L86: Note that conductivity has units of S/cm. Authors are reporting instead the resistivity (the usual value reported for millQ water in research papers) that has units of MΩ cm. Moreover authors must be careful when they represent units, it makes no sense to represent units such as MΩ/cm-1, and this would be instead MΩ/cm or MΩ cm-1. Use one unit criteria or other, not a combination of both.
7) Authors must be consistent in the way they represent units. Use “/” or superindex “-1” but not a mixture of both in the text.
8) Several subsections of the experimental description can be merged. This will results in a more comprehensive description of experimental procedures avoiding captions that contain few lines of description.
9) Experimental section should include a description of the chemical employed as well as the suppliers.
10) Can dealumination occur under acidic pH of the water samples treated?
11) L166-169: This is not a slight decrease (see Fig. 1). Can the authors further develop on the structural effect of acidification and dealumination of the zeolites?
12) Why the intense peak observed at HCLI-0.5M at 2theta 27.5º disappears at HCLI-1.0 M?
13) Crystal shapes of Fig.2a and b seem completely different. Is there any explanation for this morphological change of zeolite crystals? This is not explained in the text, where in contrast it is stated that structures are identical.
14) Why there is a change in the content of Na+, Mg2+, K+, Ca2+ and Fe2+/Fe3+. This may be deduced by the reader from explanations given in the introduction but it is not clearly explained herein. Include a statement providing an explanation for the diminution of these alkaline metals within the crystalline structure. Is ionic exchange or the removal of Al3+?
15) L185: Each cationic species must show the corresponding charge.
16) L189: XRD allows inferring a decrease in the crystallinity but not a reduction of Al content.
17) Table 1: How do the authors explain the decrease of silica at 1.0 M of H2SO4? Why the content of Ca2+ increases at 0.5 M?
18) Figure 3 shows almost identical FTIR spectra for all the zeolite tested. Which are the differences and what do they suggest? This is not clearly described in the text L195-206. Highlight which is the main conclusion derived from these results.
19) The main problem of the determination of pHpzc for the zeolites is the dealumination. How are the authors sure that after 24 h of homogenization under high acidic conditions the zeolite is not dealuminated? Considering this observation can be this method considered accurate and appropriated?
20) How do the authors explain the drastic change in the pHpzc?
21) Dealuminated material seems to have a high buffering capacity. Is there any explanation to this behavior?
22) L220-229: Experimental results seem to depict a different behavior than the described. It is not clearly described and justified the longer treatment times required to attain neither equilibrium nor the lower adsorption capacity. Authors must highlight also the main conclusion; natural zeolite has higher adsorption capacity and performance than dealuminated zeolite.
23) Authors did not analyze surface and porosity by BET. How can these differences be explained? How can be then the intra-particle diffusion model considered or excluded?
24) What is relevant from table 2? Are the constant values of models with low R2 relevant or even suitable to be considered?
25) What is the effect of dealumination on the constants?
26) What is the justification of the isotherm modification? Which mechanisms may justify this change of behavior?
27) How is justified a multilayer adsorption of Pb2+? Is a multilayer adsorption of heavy metal usually observed when using zeolites?
28) L275-276: Why this drastic removal percentage with a slight change in Si/Al ratio?
29) Is it required an experimental section of one paragraph to state that higher dose of adsorbent will result in higher removal? Which additional conclusions may be extracted from experimental results?
30) In terms of operational expenditure and adsorbent capacity which material would be the most suitable for operation?
31) L288: Lead is precipitated or hydrolyzed, but not both at the same time. Do the authors refer to the precipitation of lead hydroxide? Which one is the Ksp of these insoluble species. Moreover note that precipitation will result also in the physical separation from the aqueous phase. This does not seem to be a well-developed explanation.
32) Figure 8 shows bad fittings.
33) Why the percentages of removal observed in these graphs are completely different to previous experimental results reported in the same paper for identical adsorbents?
Author Response
Response to Reviewer 1 Comments
Comments and Suggestions for Authors
The manuscript presents a very systematic work of adsorption using zeolites. Authors do not provide a strong conclusion to highlight the meaningfulness of this study. Results are barely discussed and explanations provided are not sound. Several comments follow:
1) Tittle must highlight that the study is conducted with adsorption removal using zeolites: Si/Al ratio in natural zeolite clinoptilolite
Answer: The title was changed as follows:
Sorption of Pb(II) from aqueous solutions by acid-modified clinoptilolite-rich tuffs with different Si/Al ratios
to focus the research about the removal of Pb(II) by dealuminized materials.
2) Identify clinoptilolite as natural zeolite in the abstract.
Answer: The authors considered clinoptilolite-rich tuff instead clinoptilolite to refer the natural zeolite
3) L58-61: Can this differential structure be shown in a figure?
Answer: The International Natural Zeolite Association (http://www.inza.unina.it/) has information about the different natural zeolites and their structures and morphologies, for instance clinoptilolite, mordenite and erionite. In this paper the authors not consider these natural zeolites to develop the experimentation, for this reason only the MEB images of clinoptilolite were included.
The following images were obtained from the WEB sites of INZA.
Clinoptilotite
SEM image, Yucca Mountain, Nevada Barbara Carlos
Mordenite
SEM image, Yucca Mountain, Nevada Barbara Carlos, (LANL)
Erionite
SEM image, Shoshone, California, End-On view of Needles Robert Raymond
4) L61-62: Why it presents selective adsorption of metal cations?
Answer: The selectivity of each the natural zeolites depends on:
Nature of the cations: size and ionic charge
Temperature
Concentration of the cations in solution
Anions associated with cations in solution
Structure of the zeolite-topology and charge density of the network
5) Introduction presents a too general overview on zeolite materials. Authors are encouraged to define a clearer outline that guides the reader to the actual novelty of this research contribution.
Answer: The introduction was modified according with the reviewer suggestion. The following references were added in the new version of the manuscript.
[17] Milan Kragovic´, Aleksandra Dakovic´, Marija Markovic´, Jugoslav Krstic´, G. Diego Gattac, Nicola Rotiroti. Characterization of lead sorption by the natural and Fe(III)-modified zeolite. Applied Surface Science 283 (2013) 764–774.
[31] M. Zh Kussainova, R.M. Chernyakovа, U.Z. Jussipbekov, S. Pas. Structural investigation of raw clinoptilolite over the Pb2+ adsorption process from phosphoric acid. Journal of Molecular Structure 1184 (2019) 49-58.
6) L86: Note that conductivity has units of S/cm. Authors are reporting instead the resistivity (the usual value reported for millQ water in research papers) that has units of MΩ cm. Moreover authors must be careful when they represent units, it makes no sense to represent units such as MΩ/cm-1, and this would be instead MΩ/cm or MΩ cm-1. Use one unit criteria or other, not a combination of both.
Answer: The conductivity was changed by the resistivity in the new version of the manuscript.
7) Authors must be consistent in the way they represent units. Use “/” or superindex “-1” but not a mixture of both in the text.
Answer: The authors have homologized the representation of the units using -1 instead /, in the whole manuscript.
8) Several subsections of the experimental description can be merged. This will results in a more comprehensive description of experimental procedures avoiding captions that contain few lines of description.
Answer: The Materials and Methods section were restructured as follows:
2. Materials and methods
2.1. Clinoptilolite-rich tuff and conditioning
2.2. Samples characterization
2.3. Sorption experiments
2.3.1. Kinetic
2.3.2. Isotherms
2.4. Influence of adsorbent dosage and Effect of pH
Please see the new version of the manuscript.
9) Experimental section should include a description of the chemical employed as well as the suppliers.
Answer: A description of the chemicals employed as well as the suppliers was included in the experimental section
10) Can dealumination occur under acidic pH of the water samples treated?
Answer: There are reported papers that mention about this subject. The results found in those works suggested that the loss of aluminium from clinoptilolite occurs at pH less than 2. Therefore, the authors consider that under the experimental conditions of this work (pH from 2 to 12), the major amount of aluminium from clinoptilolite-rich tuff remains into the zeolite network after the contact with the Pb(II) solutions.
References not included in the paper
i). Chemical behavior of iron modified zeolite at different pH values of aqueous solutions. Marin Ugrina1, Nediljka Vukojević Medvidović, Jelena Perić, Marina Trgo, Aleksandra Daković, Ivona Nuić, Sonja Miličević, Živko Sekulić. Proceedings of the 3rd Croatian-Slovenian Symposium on Zeolites, 2010, 69-72.
ii). N. A. Hernández-Beltrán and M. T. Olguín. Elemental composition variability of clinoptilolite-rich tuff after the treatment with acid phosphate solutions. Hydrometalurgy 89 (3-4) 374-378, 2008.
iii). Roberto Leyva Ramos, Nahum Andrés Medellín Castillo, Rosa María Guerrero Coronado, María Selene Berber Mendoza, Antonio Aragón Piña y Araceli Jacobo Azuara. Intercambio iónico de plata(I) en solución acuosa sobre clinoptilolita. Rev. Int. Contam. Ambient. 21 (4) 193-200, 2005.
11) L166-169: This is not a slight decrease (see Fig. 1). Can the authors further develop on the structural effect of acidification and dealumination of the zeolites?
Answer: The word slight were deleted and and the following paragraph was added with the corresponding reference in the new version of the manuscript:
In this sence, Wang et al. [36] present the reaction mechansism which take place when the clinoptilolite-rich tuffs is in contact with a strong acid media (HNO3). Basically they mention the break of the primary structure of the zeolite network and the formation of Al(NO3)3. In the present work could be a similar mechanism with the formation of Al2(SO4)3.
[36] Cheng Wang, Shaozheng Leng, Huidong Guo, Liyun Cao, Jianfeng Huang Acid and alkali treatments for regulation of hydrophilicity/hydrophobicity of natural zeolite. Applied Surface Science 478, 319-326, 2019.
12) Why the intense peak observed at HCLI-0.5M at 2theta 27.5º disappears at HCLI-1.0 M?
Answer: The intense peak observed at HCL-0.5M at 2theta 27.5º can be attributed to the possible preferential orientation in the crystalline structure of the HCLI-0.5M sample.
13) Crystal shapes of Fig.2a and b seem completely different. Is there any explanation for this morphological change of zeolite crystals? This is not explained in the text, where in contrast it is stated that structures are identical.
Answer: The authors did not appreciate differences on the morphology of the clinoptilolite crystals (MEB). The same exhagonal plates were observed in all of them (after and before the acid treatment). However, the changes produced by the acid treatment were observed by other techniques.
The following lines were modified in the new version of the manuscript:
This indicates that the change in morphology of the natural zeolites with acid treatment depends on the nature as well as the crystalline structure of this zeolitic materials.
14) Why there is a change in the content of Na+, Mg2+, K+, Ca2+ and Fe2+/Fe3+. This may be deduced by the reader from explanations given in the introduction but it is not clearly explained herein. Include a statement providing an explanation for the diminution of these alkaline metals within the crystalline structure. Is ionic exchange or the removal of Al3+?
Answer: The following paragraph was added in the new version of the manuscript:
…; whereas the amount of Ca2+ and K+ ions decrease with increasing sulfuric acid concentration as result of H+ exchange or by the loss of Al3+ from the zeolite network showing that its cation exchange capacity was lost. It is clear that the H+ is preferentially exchanged by Na+ and Mg2+ from the zeolitic network than Ca2+ and K+ and this depends of the exchangeable sites that they occupy into the zeolite network [39]. It is important to mention that the Fe could be as an associated mineral (Fe2O3) of the zeolitic rock.
[39] M.Roži´c, Š. Cerjan-Stefanovi´c, S.Kurajica, M. Rožmari´c Maˇeefat, K. Margeta, A. Farkaš. Decationization and dealu mination of clinoptilolite tuff and ammonium exchange on acid-modified tuff. Journal of Colloid and Interface Science 284(1):48-56, 2005.
15) L185: Each cationic species must show the corresponding charge.
Answer: The corresponding charge were added at each cationic species.
16) L189: XRD allows inferring a decrease in the crystallinity but not a reduction of Al content.
Answer: In the new version of the manuscript this comment is supported with the addition of a reference as was mentioned before.
[36] Cheng Wang, Shaozheng Leng, Huidong Guo, Liyun Cao, Jianfeng Huang Acid and alkali treatments for regulation of hydrophilicity/hydrophobicity of natural zeolite. Applied Surface Science 478, 319-326, 2019.
17) Table 1: How do the authors explain the decrease of silica at 1.0 M of H2SO4? Why the content of Ca2+ increases at 0.5 M?
Answer: Table 1 present an average of Chemical composition (% weight) of natural and acid-treated clinoptilolites obtained at different point in the surface. The inconsistent values obtained for Ca and Si metals can be explained by the presence of points in the surface where present higher values of Ca for HCLI-0.5M and lower values for the HCLI-1.0M sample.
18) Figure 3 shows almost identical FTIR spectra for all the zeolite tested. Which are the differences and what do they suggest? This is not clearly described in the text L195-206. Highlight which is the main conclusion derived from these results.
Answer: The following paragraph and the corresponding reference were added to the new version of the manuscript.
The data obtained confirm that the dealumination of clinoptilolite takes place after the acid treatment through the band near 600 cm-1 which was modified when the clinoptilolite was treated with the H2SO4 solutions indicating the break of the bond Al-O- from the zeolite network [41].
[41] Srđan Matijašević, Snežana Zildžović, Jovica Stojanović, Marija Đošić, Jelena Nikolić, Mirjana Stojanović, Nebojša Labus. Removal of uranium (VI) from aqueous solution by acid modified zeolites. Zastita Materijala 57 (4) 551 - 558 (2016).
19) The main problem of the determination of pHpzc for the zeolites is the dealumination. How are the authors sure that after 24 h of homogenization under high acidic conditions the zeolite is not dealuminated? Considering this observation can be this method considered accurate and appropriated?
Answer: With base on the literature, the zeolite network was not dealiminated at pH between 2 and 12. The dealumination of the zeolites occurs in strong acid solutions (> 0.1 M H2SO4), as was mentioned before.
20) How do the authors explain the drastic change in the pHpzc?
Answer: The following paragraph was added to the new version of the menuscript.
In this sence, Rozic et al. [39] reported that the following reaction take placed: >Al− OH+ H+ → >AlOH2+ after the contact of the clinoptilolite-rich tuff with strong acid solutions.
21) Dealuminated material seems to have a high buffering capacity. Is there any explanation to this behavior?
Answer: The acid-modified clinoptilolite rich tuff could contain >Al-OH groups on the surface and the following reactions could take place depending of the pH of the solution in contact with the clinoptilolite-rich tuff:
>Al-OH + H+ « Al-OH2+
>Al-OH + OH- « Al-O- + H2O
22) L220-229: Experimental results seem to depict a different behavior than the described. It is not clearly described and justified the longer treatment times required to attain neither equilibrium nor the lower adsorption capacity. Authors must highlight also the main conclusion; natural zeolite has higher adsorption capacity and performance than dealuminated zeolite.
Answer: The paragraph was restructured in the new version to the manuscript to clarify the obtained results as follows.
The results presented in Figure 5b show are similar in maximum Pb(II) uptake when the Si/Al ratio ranges from 4.3 to 7.0 shows. However drastically the sorption of Pb(II) decreased as Si/Al ratio increased This can be attributed to the decrease in exchangeable ions during the acid-treatment (Table 1) and higher competition between H+ from the acid-modified clinoptilolite-rich-tuffs and Pb(II) in the solutions.
23) Authors did not analyze surface and porosity by BET. How can these differences be explained? How can be then the intra-particle diffusion model considered or excluded?
Answer: The intra-particle diffusion model was deleted in the new version of the manuscript because the experimental data not fitted well to this model and also considered the absence of the textural parameters of the no-and acid-modified clinoptilolite-rich tuffs as was commented by the reviewer.
24) What is relevant from table 2? Are the constant values of models with low R2 relevant or even suitable to be considered?
Answer: The authors consider convenient to compare the R2 from the chosen kinetics models to determined which of them could be describe the kinetic of the Pb(II) sorption for each zeolitic material.
25) What is the effect of dealumination on the constants?
Answer: The following paragraph was added to the new version of the manuscript.
The k2 diminished when the Si/Al ratio increased. However, the k1 slightly increased when the Si/Al increased as well. This results shows that the dealumination of the acid-modified clinoptilolite-rich tuff has repercussion on the Pb(II) kinetic sorption and the Si/Al ratio will determine the order of the kinetic process (pseudosecond order or pseudofirst order).
26) What is the justification of the isotherm modification? Which mechanisms may justify this change of behavior?
Answer: The following paragraph was added in the new version of the manuscript to justify the change of the Pb(II) sorption behavior by the acid-modified clinoptilolite-rich tuffs.
These differences can be attributed to the dealumination of the clinoptilolite-rich tuff after the treatment with 0.5 and 1.0 M H2SO4 solutions. The amount of aluminum in HCLI-0.5M and HCLI-1.0M were 1.7 and 2 times lower than Nat-CLI, respectively. In these cases, it is probably that the Ca2+ and K+ from the zeolitic materials had played a role on the Pb(II) sorption.
27) How is justified a multilayer adsorption of Pb2+? Is a multilayer adsorption of heavy metal usually observed when using zeolites?
Answer: The following paragraph was added in the new version of the manuscript to justify the Pb(II) sorption behavior.
It is important to mention that not a unique mechanism is involved on the sorption processes. For instance, the speciation of the heavy metal in solution, the surface charge of the sorbent as well as the nature and structure of the zeolitic materials could be some of the factors which are involved on the ion exchange or adsorption mechanisms.
Additional comment
The different models applied to the isotherms support the description of the sorption processes when the concentration of Pb(II) vary in solution. The maximum sorption capacity of the zeolitic material for Pb(II) is an important parameter which is obtained from the Langmuir model
28) L275-276: Why this drastic removal percentage with a slight change in Si/Al ratio?
Answer: The following paragraph was added in the new version of the manuscript to discuss the Pb(II) removal percentages with a slight change in Si/Al ratio.
One explanation of this behavior is the few available sorption sites in the zeolite networks of HCLI-0.5M and HCLI-1.0M to be occupy by the Pb(II) chemical species. The H-CLI1.0M has the lowest amount of Al in the zeolitic material diminishing the ion exchange capacity. The amount of Ca2+ in the zeolitic material also drastically diminished limiting the ion exchenge: Pb2+(s) + Ca2+(z) ® Pb2+(z) + Ca2+(s), where s represent the solution and z the zeolitic material. The sites where the K+ are located into the zeolite network [42] not favor the ion exchange with Pb2+. Another explanation of this behavior is by the change of the nature of the zeolitic material from hydrophilic to hydrophobic character [36].
[42] Xiaoyu Wang, Chris A. Plackowski, Anh V. Nguyen. X-ray photoelectron spectroscopic investigation into the surface effect of sulphuric acid treated natural zeolite. Powder Technology 295 (2016) 27–34.
29) Is it required an experimental section of one paragraph to state that higher dose of adsorbent will result in higher removal? Which additional conclusions may be extracted from experimental results?
Answer: In section 2.3.3. mentioned about the experimentation corresponding to the influence of the dosage of adsorbent on the Pb(II) removal by the zeolitic materials.
Additional discussion has been added about this subject on section 3.5. Please see the new version of the manuscript.
30) In terms of operational expenditure and adsorbent capacity which material would be the most suitable for operation?
Answer: The authors consider that the zeolitic material should be chosen according with the water to be treated. In acidic media with only cationic species is recommended the use of Nat-CLI. However, in a mixture of cationic and organic species, the HCLI-0.5M and HCLI-1.0M could be appropriated.
31) L288: Lead is precipitated or hydrolyzed, but not both at the same time. Do the authors refer to the precipitation of lead hydroxide? Which one is the Ksp of these insoluble species. Moreover note that precipitation will result also in the physical separation from the aqueous phase. This does not seem to be a well-developed explanation.
Answer: The paragraph was restructured to clarify the discussion of the results. Please see the new version of the manuscript.
The decrease in Pb(II) sorption at higher pH values can be explained by the formation of hydrolyzed Pb(II) species (Kps of Pb(OH)2=1.43 E-20 ) which could precipitate or by the electrostatic repulsion between a possible negative Pb(II) species (Pb(OH)3- or Pb(OH)42-) and the negatively charged surface of the HCLI-0.5M and HCLI-1.0M zeolitic materials [43].
32) Figure 8 shows bad fittings.
Answer: The Figure 8 was improved. Please see the new version of the manuscript.
33) Why the percentages of removal observed in these graphs are completely different to previous experimental results reported in the same paper for identical adsorbents?
Answer: The differences observed in the removal percentages presented on figure 8 are completely different to that observed in figure 7 due to the experimental conditions chosen in each case.
.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Authors reported the removal of lead ions by using clinoptilolite after changing the ratio of Si/Al. They observed the removal depends on the ratio, which is preferred in the region between 4 and 8 for high crystallinity. The characterization and data analysis are good, but some of figures and tables may be required to modify for better and clear view. In addition, the long conclusion needs to be rewritten, as they just summarized the results rather than they conclude the research questions and motivations, or evaluate how much this approach is important or meaningful compared to others. Some suggestions and questions are the following.
1. XRD data in Figure 1.
Authors interpreted the loss of crystallinity with the acid treatment. Ref [35] is ok, but Ref [29] is a little bit different from this manuscript. In Ref. [29] cristobalite peaks increase with the acid concentration.
2. Figure 5a)
It is recommended to change the x scale from min to hour. Do the Pb(II) updates of HCLI-0.5M and -1.0M go up slowly to meet finally others' values at around 9 to 10 after a longer time? Otherwise, are they stay there about 5 or 6 even after 1400 min?
3. It does bother that they mixed the acid concentration and the Si/Al ratios in this manuscript. Once they confirmed their Si/Al ratios in Figure 5b), their consistent use in Table 2 and Figures 6-8 would be better.
4. Effect of the solution pH
The maximum Pb(II) update percentages are 5, not 6 in Figure 8b) and 8c), even though it is mentioned as 6 on page 10, line 285.
5. Additional treatment such as 0.75M is recommended for an intermediate group in Figures 4, 5, and 6, although ti doesn't change their conclusions.
6. Many references they cited are missing with journal titles.
Author Response
Response to Reviewer 2 Comments
Authors reported the removal of lead ions by using clinoptilolite after changing the ratio of Si/Al. They observed the removal depends on the ratio, which is preferred in the region between 4 and 8 for high crystallinity. The characterization and data analysis are good, but some of figures and tables may be required to modify for better and clear view. In addition, the long conclusion needs to be rewritten, as they just summarized the results rather than they conclude the research questions and motivations, or evaluate how much this approach is important or meaningful compared to others. Some suggestions and questions are the following.
1. XRD data in Figure 1.
Authors interpreted the loss of crystallinity with the acid treatment. Ref [35] is ok, but Ref [29] is a little bit different from this manuscript. In Ref. [29] cristobalite peaks increase with the acid concentration.
Answer: In the Ref [29], it was reported that the cristobalite peaks increase with the acid concentration, however, in the references Ref [29] and Ref [35], it was mentioned that the intensity of crystallinity and peaks intensity for clinoptilolite decrease with increasing of acid concentration.
2. Figure 5a)
It is recommended to change the x scale from min to hour. Do the Pb(II) updates of HCLI-0.5M and -1.0M go up slowly to meet finally others' values at around 9 to 10 after a longer time? Otherwise, are they stay there about 5 or 6 even after 1400 min?
Answer: In the case of HCLI-0.5M and -1.0M the content of Al is lower than Nat-CLI, HCLI-0.1M and HCLI-0.2M as a consequence the ion exchange capacity drastically diminished and the sorption capacity of these dealuminated materials for lead not will reach 9 to 10 mg/g after 1400 min according with the tendency observed in figure 5a.
2. It does bother that they mixed the acid concentration and the Si/Al ratios in this manuscript. Once they confirmed their Si/Al ratios in Figure 5b), their consistent use in Table 2 and Figures 6-8 would be better.
Answer: In tables 2 and 3 a new column was included with the Si/Al ratios of each non- and acid-modified clinoptilolite-rich tuffs. Figures from 6 to 8 were modified according with the reviewer suggestion.
3. Effect of the solution pH
The maximum Pb(II) update percentages are 5, not 6 in Figure 8b) and 8c), even though it is mentioned as 6 on page 10, line 285.
Answer: The pH was corrected (5 instead 6) in the new version of the manuscript.
4. Additional treatment such as 0.75M is recommended for an intermediate group in Figures 4, 5, and 6, although it doesn't change their conclusions.
Answer: The authors appreciate the suggestion of the reviewer. However, the Pb(II) sorption behavior found for HCLI-0.5M and HCLI-1.0M are clearly different with respect the others because the dealumination of the clinoptilolite-rich tuffs.
5. Many references they cited are missing with journal titles.
Answer: The references are corrected according with the reviewer suggestion
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Authors thoroughly revised their manuscript. Acceptance is suggested.
Author Response
Sorption of Pb(II) from aqueous solutions by acid modified clinoptilolite-rich tuffs with different Si/Al
ratios
Abatal et al.
General]
Authors used clinoptilolite-rich tuffs as a sorbent to capture Pb. It is generally interesting story. Once the ms is properly revised,
I am in favor of this research!
This paper nonethelss suffer significantly from the description of adsorption performance.
Consequently, it needs to be improved significantly. As authors mainly rely on biased metrics
like max adsn capacity only, it is limiting the evaluation of the data they generated from clinoptilolite-rich tuffs against target like Pb.
Pls revise the ms significantly to allow proper evaluation of the results of all exps in a more meaningful way .
Specifics]
1] Problem of adsn capacity
For many adsorption studies, performance is generally assessed and expressed by the equilibrium (or maximum) adsorption capacity. However, it is not objective to do such judgement based on adsn capacity alone because the maximum adsorption capacity is sensitively affected by the initial loading concn of target pollutant (or more specifically what is left after sorption reaction after equilibrium).
For instance, if sorbent is exposed to higher concentration of targets, it is apt to exhibit higher adsorption capacity. On the other hand, if the sorbent is exposed to lower levels of target species, it will show lower capacities.
For a given study where all tests were done at the same intial concns, adsn capacity is useful to make comparision in relative sense only under such limited conditions.
However, it is not proper to make comparisions where studies are conducted at all different initial loading concns.
Hence, adsn capacity is not an objective metric to meaningfully assess the actual performance of sorbents.
The ideal evaluation should be made by the concept of partition coefficient (PC) as explained below. WIth the concept of PC, the use of adsn capacity will
be more meaningful in some sense.
Answer
The experimental conditions of this work were not focused to obtain the partition coefficient. In this investigation a variable concentrations of Pb(II) in solution were used to obtain the isotherms as a consequently the maximum sorption capacity of the acid-modified rich-tuffs for Pb(II) applied the Langmuir model.
2] Raw isotherm data as well across varying breakthrough levels (not only 100% but some early BT points)
a] To resolve this problem, pls prepare a new Table to show some specifics of the performance data. Authors show only estiamted Qe (max capacity) values
in Table. Pls show the measured Qe values at 100% BT (I assume that authors already did). In addition, pls show the results at varying BT levels such as 5, 10, and 50%
BT level as well in addition to 100% of equilibrium which authors did if possible. If authors cannot show the data for all other than 100%, pls try to show
the adsn capacity and PC at least at 10% BT point (in addition to 100%). Authors may show all detailed experimental data of adsorption capacity in Table as they are measured in addition to estimated values from model fit.
Answer
The experimentation of the present work was not carried out in column system considering a determined flux of the influent (water with a determined concentration of Pb(II)) to obtain the breaktrough curves.
3] Computation of PC
To make a more objective comparison of performance between different sorbents or data obtained under different conditions,
pls compute the partition coefficient (PC: or distribution coefficient (DC): Here I mean both PC and DC as the same concept) and use it for assessing the actual performance.
Pls compare the data with previous study using PC values and expand discussion.
Pls compute PC values and use them along with adsorption capacity to evaluate the performance: Create a new
Table (or improve existing Table) to show authors' experimental more specifically by including such variables as Initical concn of target, final concn of target, breakthrough (BT: e.g., 10% and 100%),
adsorption capacity (mg/g), partition coefficient (mg/g/uM). (Although authors can use 100% BT, 10% BT is more ideal to define the capacity of sorbent. This is because
the state of saturation is diagnosed more efficiently in early BT point as explained by Sjulejko et al. (in ref shown below).)
PC can be computed as follows:
PC=Adsorption capacity/Final concentration (or = Adsorption capacity/(Initial concn*removal rate)).
The resulting data can be expressed as "mole/Kg/Pa" for gas phase sorbent or as "mg/g/uM for liq phase sorbent (<- Refer to refs).
All PC values should be presented in Table and related discussion should also be provided in the main text.
Answer
The experimentation of the present work was carried out in batch system with variable concentrations of lead (Pb(NO3)2) in solutions to obtain the Langmuir isotherms which parameter qm represents the maximum sorption capacity of the acid-modified rich tuffs. In these cases the Partition Coefficient was not obtained under the experimental conditions of this work.
It is important to mention that the authors applied the liner model in the cases of the sorption of Pb(II) by HCLI-0.5 and HCLI-1.0 to obtain the Kd values which represent the distribution coefficients for comparison purpose between them.
Kd= 0.0304 L/g
Kd= 0.0221 L/g
4] For details of PC computation, authors may refer to some of the proper reference if needed: Authors may or may not necessary cite the following refs but take this concept thoroughly and properly to make a full revision of ms to accommodate the PC to properly judge the performance of materials proposed in this study relative to other materials introduced previously. (Down below are just examples of papers showing the concept of PC. There is no obligation for authors
to cite these! Authors may find and use some other references if they can find some other relevant references.)
- Al-Wabel M., Elfaki J., Usman A., Hussain Q., OK YS (2019)
Performance of dry water- and porous carbon-based sorbents for carbon dioxide capture. Environ Res. 174, 69-79. <<- Gas phase
- Na C.-J., Yoo M.-J., Tsang D.C.W., Kim H.W., Kim K.-H. (2019) High-performance materials for effective sorptive removal of
formaldehyde in air. Journal of Hazardous Materials. 366, 452-465. <<- Gas phase
-Szulejko J.E., Kim K.-H, Parise J. (2019) Seeking the most powerful and practical real-world sorbents for gaseous benzene as a
representative volatile organic compound based on performance metrics. Separation and Purification Technology 212, 980-985. <<- Gas phase
- Vikrant K., Kim K.-H. (2019) Nanomaterials for the adsorptive treatment of Hg(II) ions from water. Chemical Engineering Journal.
358, 264-282. <<- Liq phase
Answer
The authors revised the papers suggested by the reviewer. However, not in all cases the initial concentration was the same to obtain the maximum sorption capacities. Therefore the calculation of the kd will not follow the equation mentioned before.
5] Based on new edition of PC in Tables (authors are asked to create new Tables to show details of their experiment first. In addition, additional Table
should be made to allow performance comparison between theirs and others), authors are asked to make discussion, especially performance evaluation based on
PC values in R & D section.
Answer
The experimental conditions of this work were not focused to obtain the partition coefficient. In this investigation a variable concentrations of Pb(II) in solution were used to obtain the isotherms as a consequently the maximum sorption capacity of the acid-modified rich-tuffs for Pb(II) applied the Langmuir model.
6] Authors are also asked to create a new Table (as mentioned above) to allow comparison of performance in terms of PC and many other performance metrics discussed above
between this work and many previous studies. In Table S2, pls add more parameters (performance metrics) for more meaningful comparison such as
initial loading concn, final concn (or equil concn), pH (if exps are for liq phase), adsn capacity (at 100% BT and low BT (5 or 10%) as well), PC (at 100% BT + at low BT (5 or 10%)), and so on.
Authors should then tell the real-world performance based on PC as a means to explain biased patterns derived by adsorption capacity data.
Answer
The experimentation of the present work was carried out in batch system with variable concentrations of lead (Pb(NO3)2) in solutions to obtain the Langmuir isotherms which parameter qm represents the maximum sorption capacity of the acid-modified rich tuffs. In these cases, the Partition Coefficient was not obtained under the experimental conditions of this work.
7] Especially, the results shown in the updated Table should
be evaluated based on PC along with Adsn capacities at the same time to judge the performance by two contrasting criteria (Adsn capacity vs. PC).
In general, most authors have been comparing to determine the best sorbent simply based on adsn capacity which can lead to extremely
biased judgement. Such wrong biased comparison based on Adsn capacity then should be
criticized by the new ordering set by more objective criteria of PC! The wrong order by adsn capacity can be critically assessed by the more objective
criteria of PC! So it will be ideal to assess the performance based on Adsn capacity first. And then expand discussion how those results are biased
based on PC values derived newly and shown in revised Table(s).
Answer
The experimentation of the present work was carried out in batch system with variable concentrations of lead (Pb(NO3)2) in solutions to obtain the Langmuir isotherms which parameter qm represents the maximum sorption capacity of the acid-modified rich tuffs. In these cases the Partition Coefficient was not obtained under the experimental conditions of this work.
8] Authors may also provide discussion wrt Figure of Merit (FoM) of the composites they generated.
Answer
Under the experimental conditions of this work Figure Merit was not obtained.
9] Provide detailed QA for measurements of target species before and after the treatment.
Answer
The measurements (QA) were not considered in the present work.
10] In the revised Abs, now pls describe both PC and Adsn capacity in relation with BT level (e.g., 10% and 100%).
Further, information on initial loading concn should also be provided. To conclude, BT level and such information should be given in Abstract.
Answer
The experimentation of the present work was carried out in batch system with variable concentrations of lead (Pb(NO3)2) in solutions to obtain the Langmuir isotherms which parameter qm represents the maximum sorption capacity of the acid-modified rich tuffs. In these cases the Partition Coefficient was not obtained under the experimental conditions of this work.
11] BT level and initial concn information should also be given when you describe the results in discussion section!
Answer
The experimentation of the present work was carried out in batch system with variable concentrations of lead (Pb(NO3)2) in solutions to obtain the Langmuir isotherms which parameter qm represents the maximum sorption capacity of the acid-modified rich tuffs. In these cases the Partition Coefficient was not obtained under the experimental conditions of this work.
Author Response File: Author Response.docx
